EP 1 857 825 A1 and the parallel document U.S. Pat. No. 7,576,547 B2 disclose a measuring arrangement comprising a grounding point for determination of the insulation resistance of a live electrical apparatus or of an installation with a supply voltage with a positive terminal and a negative terminal. In this case, two switches or a corresponding changeover switch are provided, which each produce a current path between one of the two terminals and a grounding point in order to determine the total insulation resistance that results in the event of the occurrence of one or more insulation faults with any arbitrary potential reference. In order to determine the insulation resistance, two successive measurements are performed, wherein, in the first of these measurements, the first switch is closed while the second switch is open, and wherein, in the second of these measurements, the first switch is open while the second switch is closed. During the measurements, the current flowing via the respectively produced connection to the grounding point is measured. The insulation resistance is calculated from the two measured currents, in respect of which the cited documents provide a detailed instruction.
The known measuring arrangement can be provided specifically for monitoring a photovoltaic system that feeds electrical energy from a photovoltaic generator into an AC grid for an insulation fault. The component parts to be provided for forming the known measuring arrangement at an inverter are numerous and increase the total costs of the inverter considerably.
DE 10 2006 031 663 B3 discloses a method for measuring the insulation resistance in an IT network. This IT network has a DC link and a self-commutated converter comprising a first and a second power switch. Furthermore, a measuring arrangement for measuring of the DC link voltage with respect to ground consisting of a voltage divider and two associated potential measuring devices is part of the IT network. The method comprises offline measurement, for example at the beginning of operation, in which all of the first or all of the second power switches of the converter are closed. In this switching state, the two potentials of the DC link and the DC link voltage are measured and the insulation resistance is determined from this.
DE 10 2010 054 413 A1 discloses a method for localizing an insulation fault in a system that has a DC section with a high side and a low side and an AC section including an inverter with a series circuit comprising two power switches that are connected between the high side and the low side. A DC voltage from a DC voltage source is supplied to the DC section. The power switch present directly on the high side is turned on, the power switch present directly on the low side is turned on, in each case a high-side insulation voltage between the high side and a ground and a low-side insulation voltage between the low side and ground are measured in each of the two on states and finally, on the basis of the measurement results, it is determined whether there is an insulation fault in the DC or AC section.
DE 10 2011 007 222 A1, which was not published until after the priority date of the present application, discloses an inverter circuit in which, for insulation monitoring, a voltage divider ratio of a voltage divider used for insulation monitoring is changed by means of AC voltage shunts of the inverter by virtue of said shunts being coupled to DC link potentials by means of IGBTs in a bridge circuit. During normal operation of the inverter, the voltage present at the AC voltage shunts represents the instantaneous value of the output voltage and is used for regulation thereof. The insulation monitoring takes place by virtue of voltage measurement of an actual voltage in the form of a potential difference between a DC link potential and ground. The actual voltage is compared with a preset setpoint voltage that is dependent on the switch position of the IGBTs. An insulation fault is indicated when a difference between the actual voltage and the setpoint voltage exceeds a preset degree.
There still is a need of a method of measuring an insulation resistance for an inverter and an inverter comprising an apparatus for measuring an insulation resistance that use active measurement of the insulation resistance without incurring high costs for this.